The invention relates to the electronic assemblies associated with capacitive keyboards for identifying the keys which are depressed on these keyboards.
More precisely, it relates to the assemblies of the above mentioned kind for reading out, with a view to the subsequent use thereof, the information elaborated i.e., outputted by means of keyboards comprising a plurality of keys of the capacitive type distributed in a matrix arrangement i.e. disposed at the intersection points of a checkerboard pattern formed of m lines and n columns, m and n being whole numbers generally between 8 and 16.
In the keyboards in question, each key is associated with a capacitor and depression thereof results in a considerable variation of the capacity of this capacitor.
The two electrodes of each capacitor corresponding to the line i and to the column j (i being a whole number between l and m and j being a whole number between l and n) are connected respectively, the first one to the first electrodes of the other capacitors of line i and the second one to the second electrodes of the other capacitors of column j.
Depression of the key associated with such a capacitor then results in "capacitively coupling" line i and column j together.
For identifying each key depressed in such a keyboard, means are provided for successively applying to the inputs of the m lines trains of n successive identical pulses spaced apart mutually in time by identical times t, means for successively collecting at the outputs of the n columns the signals created by the above pulses, the n outputs being for this scanned successively at timing t and this scanning being renewed m times in succession, and means for detecting and identifying, among the signals collected, the echoes which exceed a predetermined threshold and which correspond to depressed keys.
If only the key corresonding to line i and to column j of the keyboard is depressed at a given moment, the only echo detected and identified by the above described exploration corresponds to the pulse of rank j of the train of rank i.
The identification assemblies of the above kind are satisfactory when the capacitor variations of the capacitors associated with the keys are sufficiently large with respect to the parasite coupling capacitances existing between the lines and the columns.
But this is no longer the case when these parasite capacitances are relatively high to the extent of being of the same order of size as the variations considered, said capacitances and variations being for example both of the order of 10 pF.
In such a case, in fact, the above described identification echoes, which correspond to keys of the keyboard actually depressed at a given time, do not differ sufficiently from the parasite signals created at the same time at the level of some at least of the keys which are not depressed.
It then becomes hazardous to entrust to the collecting means the mission of identifying the echoes without any other precaution.
These means are in fact sensitive only to the high or low logic levels of the electric signals representative of these echoes and not to the true amplitudes of said signals.
To overcome this drawback, it has already been proposed to adjust the detection level of the signals in question by comparing each of them, in an appropriate comparator, to a reference voltage determined as a function of the characteristics of the keyboard, during manufacture thereof; said signals and said reference are received respectively at the two inputs of the comparator and only the signals are selected whose amplitude exceeds that of the reference.
These assemblies form an improvement but do not give satisfaction in all cases.
In fact, the value of the reference is determined therein once and for all and does not remain ideal or even appropriate in all circumstances. Thus, some external parasites, particularly of the electromagnetic types, may simultaneously affect the state of the whole of the keys of the keyboard to the extent of unduly increasing at a given time the output signal collected from the column worked at that time and of causing this signal to be interrupted as corresponding to a depressed key even if none of the keys of this column has been depressed at that time.
To overcome this drawback, it has already been proposed to adjust the compensation reference at all times as a function of the actual state at that time of the different keys forming at least two columns of the keyboard and, for this, to give to said reference an amplitude M related to the mean of the amplitudes of the signals collected at said time from said columns.
This method forms an improvement with respect to the preceding one for the case where it is desired to protect the keyboard from relatively intense parasites of short duration.
But is does not allow certain troubles to be compensated for such as the one resulting from a plurality of keys of the keyboard disposed in the same line being held simultaneously depressed.
In fact, in such a case, the mean M of the amplitudes of the signals collected at the different columns corresponding to these keys itself varies at the timing of the pulses applied to the line considered and it may happen that a useful signal intended for identifying the depression of a key of another line no longer differs itself sufficiently from the reference M elaborated at the same time for being truly identified as "useful".
The invention overcomes this drawback and others of the same kind.